1. Field of the Invention
The present invention is related to explosive compositions such as ammonium nitrate/fuel oil (ANFO) and heavy ANFO type explosives, and in particular to a thermal treatment to create pores by modifying the crystalline structure of high density ammonium nitrate in order to manufacture ANFO and heavy ANFO.
2. Description of the Related Art
Ammonium nitrate/fuel oil (ANFO) type explosives are a mixture of ammonium nitrate and fuel oil, which are used as a blasting compound in mining and industrial engineering. Typically, ANFO type explosives are composed of 94% ammonium nitrate and 6% fuel oil, and have a density of 0.8-0.9 kg/L. The ammonium nitrate particles used for ANFO type explosives are porous and spherical in shape because their microporous structure of air cavities enclosed within the body of the prills provide a larger number of points of initiation with increased detonation sensitivity, or hotspots, which are closed adiabatically as a result of mechanical action and spread the burning process throughout the charge. The fuel oil is absorbed by the ammonium nitrate particles to produce a free-flowing particulate mixture which can be detonated. Other additives may be added to this mixture in order to modify the properties of the ANFO explosive, such as adding guar gums and polyisobutylene to improve water resistance.
Detonation velocity of ANFO explosives falls within the range 2500-3500 m/s because of its volumetric density and flame temperature, which are lower than that of other industrial explosives. The sensitivity of these materials to detonation is also lower than that of emulsion and dynamite type explosives; thus, the handling of ANFO type explosives is easier. The performance of ANFO depends on the ammonium nitrate prills used and the efficiency of mixing the prills with the fuel oil.
Heavy ANFO type explosives comprise a mixture of bulk emulsion and ammonium nitrate/fuel oil (ANFO). The use of emulsion type explosives as a coating substance creates a waterproof-resistant barrier, surrounding the particles of ANFO and solving the problem of its low water resistance. This waterproof-resistant barrier also improves other characteristics of ANFO by increasing its density, detonation velocity, sensitivity to initiation and shock wave intensity. Heavy ANFO is typically prepared in a bulk truck by making ANFO first, and then blending it with emulsion. Bulk emulsion used in the preparation of heavy ANFO may be gassed or not gassed. The system of heavy ANFO allows for a great deal of flexibility in the relative proportions of ANFO/bulk emulsion. This ratio can be optimized depending on the blast site-specific requirements.
As mentioned, ANFO and heavy ANFO type explosives typically need porous ammonium nitrate prills. Previous attempts to use high density ammonium nitrate in ANFO and heavy ANFO manufacturing include U.S. Pat. No. 5,240,524. This invention provides a method for modifying ammonium nitrate's density that comprises mixing it with a liquid medium, such as water, nitric acid or sodium nitrite, which penetrate the ammonium nitrate particles via pre-existing pathways, dissolving ammonium nitrate and producing a gassing reaction. As a result, the method obtains particles of higher porosity. However, the present invention differs from the aforementioned patent in the method used to create those pores.
The present invention provides a method to prepare ANFO and heavy ANFO type explosives by using high density ammonium nitrate, preferably fertilizer or technical grade, in which the crystalline structure is modified by a thermal treatment process, as a substitute for porous ammonium nitrate.
Information about the Assignee
EXSA was incorporated in 1954. The company's plant, offices and main warehouses are located in Lima, Peru. In addition, the company has various business offices, other industrial plants, powder magazines, and warehouses throughout the Peruvian territory.
EXSA engages in the manufacture, transformation, industrial operation, representation, development, research, marketing, distribution, transportation, import and export of explosives, as well as their components, accessories, associated products and by-products. Likewise, EXSA may provide any services associated with the aforementioned activities, including specialized support works for mining prospecting, development and operation, and ore reduction.